1. Field of the Invention
The present invention relates to a damper for washing machines, and more particularly to a damper for use in a drum type washing machine, which causes different damping forces according to the vibration degrees of a tub mounted inside the washing machine, thereby effectively attenuating the vibration of the tub.
2. Description of the Related Art
In general, a drum type washing machine comprises a drum horizontally installed therein for loading the laundry, washing water and detergent, and is adapted to perform washing of the laundry by making use of friction between the laundry and the drum as the drum is rotated by the driving force of a motor. Such a drum type washing machine almost never causes damage or entanglement of the laundry, and provides good washing effects of the same quality as if the laundry were washed by hand.
FIG. 1 is a sectional view illustrating a conventional drum type washing machine, and FIG. 2 is a longitudinal sectional view illustrating a conventional damper for use in the drum type washing machine as shown in FIG. 1.
As shown in FIG. 1, the drum type washing machine comprises a cabinet 2 defining the external appearance thereof, a base 2a formed as the bottom portion of the cabinet 2, a tub 6 elastically supported within the cabinet 2 by means of a spring 4 and a damper 10, a drum 7 rotatably installed inside the tub 6 and configured to accommodate the laundry and washing water for performing washing of the laundry, and a motor 8 installed behind the tub 6 so that it is connected to the drum 7 for rotating the drum 7.
Above the tub 6 there are installed a water supply valve assembly 13 and a detergent box assembly 15 for supplying the washing water and detergent into the tub 6 and the drum 7. Under the tub 6 there is installed a drainage valve assembly 17 for draining the used washing water from the tub 6 and the drum 7.
The tub 6 is installed in such a manner that the upper side portion thereof is suspended within the cabinet 2 by means of the spring 4, and the lower side portion thereof is supported at the upper side of the base 2a by means of the damper 10. With this installed configuration, the tub 6 is less affected by vibration caused as the drum 7 is rotated.
Referring to FIG. 2, the damper 10 comprises a cylinder 12 connected at the upper end thereof to the lower end of the tub 6 by means of a hinge, a piston 14 movably inserted inside the cylinder 12 and connected at the lower end thereof to the upper side of the base 2a by means of a hinge, and a friction member 16 provided around the outer peripheral surface of the piston 14 so as to come into close contact with the inner peripheral surface of the cylinder 12.
The piston 14 is formed with a pair of upper and lower stoppers 14a and 14b, which protrude radially from the outer peripheral surface thereof for supporting the friction member 16 at both upper and lower sides of the friction member 16.
The friction member 16 is configured so as to generate a constant damping force, which is set in proportion to the vibration degrees of the tub 6 estimated taking into account of the driving speed of the motor 8, and the like. By virtue of the fact that the friction member 16 is configured to generate such a constant damping force while coming into close contact with the inner peripheral surface of the cylinder 12, the friction member 16 serves to cause frictional heat energy inside the cylinder 12 when the piston 14 moves relative to the cylinder 12 due to the vibration of the tub 6, thereby acting to attenuate the vibration of the tub 6.
FIGS. 3 and 4 are graphs illustrating waveforms of force transmitted through the conventional damper configured as stated above. More particularly, FIG. 3 is a graph illustrating waveforms of the transmitted force when normal vibration is applied to the damper during a dehydrating cycle, and FIG. 4 is a graph illustrating waveforms of the transmitted force when excessive vibration is applied to the damper during the dehydrating cycle.
The function of the damper realizes an attenuation in the transmission of vibration force by virtue of its frictional force. With the use of the conventional damper for washing machines, due to the characteristics of frictional phenomenon, the vibration force F to be transmitted to the base 2a of the washing machine takes the form of rectangular waveforms as shown in FIGS. 3 and 4. Further, the vibration force F to be transmitted to the base 2a has a frequency characteristic causing a peak at all frequencies corresponding to harmonics of driving frequencies. This increases a possibility of generation of resonance with other peripheral structures including the base 2a and the like, resulting in adverse effects to the vibration/noise of the washing machines.
That is, as shown in FIG. 3, even in a normal state wherein the motor 8 is rotated at a constant speed during the dehydrating cycle, the tub 6 vibrates within a relatively small vibration width. In an inordinate state wherein the motor 8 is rotated at excessively accelerated or reduced speed during the dehydrating cycle, as shown in FIG. 4, the vibration width of the tub 6 becomes relatively large. As can be seen from the above description, there is a problem in that it is impossible to effectively attenuate the vibration of the tub 6 in such an inordinate state, even including the normal state, during the dehydrating cycle.